A proton exchange membrane fuel cell (PEMFC) is also referred to as a high molecular membrane fuel cell, a basic structure of which is a monocell consisting of a membrane electrode assembly (MEA) disposed among two pieces of bipolar plates, a current collector, and two end plates. Two opposite sides divided by the PEMFC membrane electrode assembly are an anode having hydrogen or a rearrangement gas, and a cathode having oxygen or air. At the anode, there is an oxidation reaction, and the cathode a reduction reaction. When the hydrogen at the anode side contacts with a catalyst, which is usually platinum or a platinum alloy, adjoining to the proton exchange membrane, the hydrogen molecule decomposes into hydrogen ion and electron. The electron would move toward the cathode side via a bridge interconnecting the anode and the cathode and a load serially connected to the bridge. On the other hand, the hydrogen ion directly passes through the proton exchange membrane to reach the cathode from the anode. It is noted the proton exchange membrane is a moisture-laden membrane and has the property of allowing only the hydrogen ion together with water molecule to pass therethrough while other gas molecules are blocked. Under an action of the catalyst at the cathode side, the electron reaches the cathode via the bridge bonds with the oxygen to produce oxygen ion, which then bonds with the hydrogen ion passed through the proton exchange membrane to form water molecule. This is the so-called electrochemical oxidation and reduction reaction process.
The monocell of PEMFC generally has an operating voltage from 0.6V to 0.9V, which is too small to actuate most electric apparatus. Therefore, in practical application, it is necessary to serially connect a plurality of monocells to form a fuel cell bank to increase the cell voltage. Each fuel cell bank includes at least two monocells. The number of serially connected monocells depends on actual voltage requirement. The fuel cell bank has a total voltage equal to a sum of voltages of all monocells. According to general electricity principle, the serially connected monocells in the fuel cell bank have the same current flown therethrough, but not necessarily have the same voltage. Under an ideal condition, the monocells in a fuel cell bank should have the same voltage. However, from the viewpoints of thermodynamics and kinetics, due to the temperature distribution in different zones and the flowing of reactant gas, it is unable to ensure that all the monocells could operate under identical conditions. As a result, the monocells have different voltages from one another. Moreover, errors produced in the process of assembling the fuel cell bank would possibly cause gas leakage in some of the monocells or the phenomenon of crossover of gas, which would also result in voltage difference among monocells. Therefore, it is possible to know whether there is any problem in the process of assembling the fuel cell bank from the voltage values of monocells included in the fuel cell bank. That is, the voltage values of monocells may be used as a basis for diagnosing a fuel cell bank. Meanwhile, the failure of any one of the monocells in the fuel cell bank is deemed as an open circuit possibly causing the failure of the whole fuel cell bank. Therefore, the voltage measurement for monocell is very important to the performance of the fuel cell bank.
U.S. Pat. No. 6,724,194 & 6,410,176; WO 2004/051773; CA 2277397 & 2454782; DE 10334556; and JP 2003086205, 2004079192 & 2004087163 disclose several means of measuring the voltage of a fuel cell. Most of these prior art structures have fixed or specific applications and are not readily applicable to general fuel cell voltage measurement.